LibJS: Extract most of Interpreter's run() into execute_statement()

Interpreter::run() was so far being used both as the "public API entry
point" for running a JS::Program as well as internally to execute
JS::Statement|s of all kinds - this is now more distinctly separated.
A program as returned by the parser is still going through run(), which
is responsible for creating the initial global call frame, but all other
statements are executed via execute_statement() directly.

Fixes #3437, a regression introduced by adding ASSERT(!exception()) to
run() without considering the effects that would have on internal usage.

Libraries/LibJS/Tests/exception-in-catch-block.js

@@ -0,0 +1,25 @@
+test("Issue #1992, exception thrown in catch {} block", () => {
+    var tryHasBeenExecuted = false;
+    var catchHasBeenExecuted = false;
+    var finallyHasBeenExecuted = false;
+    expect(() => {
+        try {
+            tryHasBeenExecuted = true;
+            foo();
+            // execution must not reach this step
+            expect().fail();
+        } catch (e) {
+            catchHasBeenExecuted = true;
+            bar();
+            // ...also not this step
+            expect().fail();
+        } finally {
+            finallyHasBeenExecuted = true;
+        }
+        // ...or this step
+        expect().fail();
+    }).toThrow(ReferenceError, "'bar' is not defined");
+    expect(tryHasBeenExecuted).toBeTrue();
+    expect(catchHasBeenExecuted).toBeTrue();
+    expect(finallyHasBeenExecuted).toBeTrue();
+});